Users to Events is many-to-many for membership. The way you have your Events table laid out, it's one-to-many for ownership, but you may want to re-think that -- it may be useful to allow Events to have more than one Owner.

Users get accepted or denied for Membership, not for Ownership, right? There is no direct relationship between Membership and Ownership. Both Membership and Ownership define relationships between Users and Events, and both of those (in my design) are many-to-many. If each group is to have only one owner, then you don't need the Ownership table. Instead, you have an Owner field on the Events table, as you originally designed. I think it's useful to allow multiple Owners (if this is a real-world project). It can be dangerous to have all administrative power in one user's hands. What if that user dies, or quits, or just forgets his password?